Cartmans dad was told in episode 201, which was taken off air but i had my ways of watching it...it was said that cartmans ad was Scott Tenormans dad who was on the Denver Broncos, so Eric killed his dad.

When Terrance & Phillip are watching American TV, they flip to SP for a minute. As pointed out above, part of it was cut, but you hear:Mephesto: "The father of Eric Cartman is..."Cartman: "HE'S my father??"